This invention relates to an air-fuel ratio control system capable of controlling the air-fuel ratio accurately regardless of changes in characteristics of oxygen concentration detectors in the case of negative feedback control of the air-fuel ratio by detecting the oxygen concentration in the exhaust gas for purifying the exhaust gas of an internal combustion engine.
The purifying ability of a catalyst is very high when the air-fuel ratio of the mixture gas is at or near a predetermined (stoichiometric) air-fuel ratio (air excess rate .lambda.=1). In view of this, in a conventional system suggested for purifying the exhaust gas by use of a three-way catalyst in the exhaust system of the internal combustion engine, the oxygen concentration in the exhaust gas representing the air-fuel ratio of the mixture gas is detected by an oxygen concentration detector (which may hereinafter be called the oxygen sensor) and the air-fuel ratio of the mixture gas is controlled to about the stoichiometric value by negative feedback. The output characteristics of this oxygen sensor, however, greatly vary with time or according to the production processes. In the case where this oxygen sensor is placed upstream of the three-way catalyst in the exhaust system of an internal combustion engine, the output characteristics thereof in relation to the air-fuel ratio vary, for example, between the oxygen sensors S1 and S2 as shown in FIG. 1A. Therefore, even if a reference level Vs is set corresponding to the target stoichiometric value (.lambda.=1) for negative feedback control, and accurate control of the stoichiometric air-fuel ratio may be attained for the oxygen sensor S1, while an air-fuel ratio lower than the stoichiometric value is attained for the other oxygen sensor S2, thus resulting in the loss of the effect of the three-way catalyst.
In the case where the oxygen sensor is disposed downstream of the three-way catalyst, on the other hand, it was confirmed that as shown in FIG. 1B, the curves of the detection voltage characteristics of the oxygen sensors S1 and S2 with respect to the air-fuel ratio pass the set reference level Vs at or near the stoichiometric air-fuel ratio. In this way, the oxygen sensor may be placed downstream of the three-way catalyst for negative feedback control of the air-fuel ratio, but in this case it is apparent that a system response delay occurs unlike in the case where the oxygen sensor is placed upstream of the three-way catalyst.
In order to obviate this problem, the present inventors suggested earlier a system having a first oxygen sensor provided upstream of the three-way catalyst in the exhaust system of an internal combustion engine for negative feedback control of the air-fuel ratio, in which a second oxygen sensor is placed downstream of the three-way catalyst and the reference level for comparison with the detection signal of the first oxygen sensor is regulated by the detection signal of the second oxygen sensor, thus compensating for the lack of uniformity of output characteristics of the first oxygen sensor. It has been found, however, that in this system controlling the reference level, the feedback control system is not sufficiently stable to be used in actual application.